When a resonator filter is manufactured, its transmission characteristics, i.e. its frequency response, must be arranged to comply with the requirements. This requires that the strengths of the couplings between the resonators are correct and that the resonance frequency, or natural frequency, of each resonator has a pre-determined value especially in relation to the natural frequencies of other resonators. In serial production, the variation of the natural frequency of a certain resonator of different filters is generally too wide with regard to the filter requirements. Because of this, each resonator in each filter must be tuned individually. Tuning like this is here called the basic tuning. A very common resonator type in filters is a coaxial quarter-wave resonator, which is shorted at its lower end and open at its upper end. In that case the basic tuning can be performed, for example, by turning the tuning screws on the cover of the filter housing at the inner conductors of the resonators or by bending the protruding parts of the extensions formed at the ends of the inner conductors. In both cases, the capacitance between the inner conductor and the cover changes in each resonator, in which case the electric length and natural frequency of the resonator also change.
When the filter is intended to be part of a system in which a division of the transmitting and receiving bands into subbands is used, the width of the passband of the filter must be the same as the width of a subband. In addition, the passband of the filter must be arranged at the desired subband. In principle, this can take place already at the manufacturing stage in connection with the basic tuning. However, in practice often a certain standard basic tuning only is carried out at the manufacturing stage, and the subband is selected in connection with taking into use by shifting the passband of the filter when required. The passband is shifted by changing the natural frequencies of the resonators by the same amount without touching the couplings between the resonators.
The natural frequencies of the resonators can be changed for shifting the passband by tuning each resonator separately and by watching the response curve. However, such adjustment is time-consuming and relatively expensive, because tuning has to be implemented manually in several iteration steps in order to achieve the desired frequency response. FIG. 1a,b presents a resonator filter known by the applicant from the application FI20030402, the passband of which can be shifted by a one-time adjustment. The filter 100 is a six-resonator duplex filter. The cover, bottom, side walls and end walls form a conductive filter housing, the inner space of which has been divided by partition walls into resonator cavities. In FIG. 1a, the structure is seen from above as the cover removed. The resonators are coaxial quarter-wave resonators; each of them has an inner conductor, the lower end of which is galvanically coupled to the bottom and the upper end of which is “in the air”. The resonators are in two rows of three resonators. The first 110, the second 120 and the third 130 resonator form a transmitting filter, and the fourth 140, the fifth 150 and the sixth 160 resonator form a receiving filter. The third and the fourth resonator are parallel in the 2×3 matrix, and they both have a coupling to the antenna connector ANT. The sixth resonator has a coupling to the receiving connector RXC and the first resonator to the transmitting connector TXC. In the transmitting and receiving filter, there is an electromagnetic coupling between the resonators through openings in the partition walls, for example.
For adjusting the filter, the structure includes a united dielectric tuning piece, which consists of resonator-specific tuning elements, such as the tuning element 128 of the second resonator and the tuning element 148 of the fourth resonator, and an arm part 108. The arm part has the shape of a rectangular letter U; it has a first portion extending from the first to the third resonator, a transverse second portion extending from the third to the fourth resonator, and a third portion extending from the fourth to the sixth resonator. Each resonator-specific tuning element is, in a way, an extension of the arm part of the tuning piece. The united tuning piece can be moved horizontally in the longitudinal direction of the filter back and forth so that the tuning elements move to a position above the inner conductors of the resonators or away from a position above the inner conductors. The moving takes place either through a slot in the cover or an opening at the end of the filter housing on the side of the third and the fourth resonator. When at the left limit of the tuning range, each tuning element is above the inner conductor of the resonator, and when at the right limit of the tuning range, each tuning element is beside the inner conductor of the resonator as viewed from above. In the former case, the effective dielectric coefficient in the upper part of the resonator cavity is at the highest, because the dielectric element is located in a place where the strength of the electric field is at the highest when the structure is resonating. Then the capacitance between the upper end of the inner conductor and the conductive surfaces faces around it is at the highest, the electric length of the resonator at the highest and the natural frequency at the lowest. Correspondingly, when the tuning element is at the right limit of its adjusting range, the natural frequency of the resonator is at the highest.
In FIG. 1b the cover 105 of the filter 100 and the tuning piece are seen from the side. The arm part 108 of the tuning piece runs through notches in the upper edge of the partition walls of the resonators, keeping the whole tuning piece against the lower surface of the cover. In the example of the figure, the tuning elements reach deeper into the resonators in the vertical direction than the arm part of the tuning piece. For example, the tuning element 128 of the second resonator extends close to the upper end of the second inner conductor 121, drawn in the figure.
In the filter shown by FIGS. 1a,b, both the transmitting and receiving band shift by a one-time adjustment because of the unity of the tuning piece. The structure is relatively compact, but moving the tuning piece requires a bit of mechanism.